1. Field of the Invention
This invention relates to the packaging of opto-electronic semiconductor arrays, specifically including vertical cavity surface emitting laser arrays and photodetector arrays.
2. Discussion of the Related Art
Vertical cavity surface emitting lasers (VCSELs) represent a relatively new class of semiconductor lasers. While there are many variations of VCSELs, one common characteristic is that they emit light perpendicular to a wafer's surface. Advantageously, VCSELs can be formed from a wide range of material systems to produce specific device characteristics. In particular, the various material systems can be tailored to emit different wavelengths, such as 1550 nm, 1310 nm, 850 nm, 670 nm, and so on.
VCSELs include semiconductor active regions, distributed Bragg reflector (DBR) mirrors, current confinement structures, substrates, and contacts. Because of their complicated structure, and because of their material requirements, VCSELs are usually grown using metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE).
FIG. 1 illustrates a typical VCSEL 10. As shown, an n-doped gallium arsenide (GaAs) substrate 12 has an n-type electrical contact 14. An n-doped lower mirror stack 16 (a DBR) is on the substrate 12, and an n-type graded-index lower spacer 18 is disposed over the lower mirror stack 16. An active region 20, usually having a number of quantum wells, is formed over the lower spacer 18. A p-type graded-index top spacer 22 (another confinement layer) is disposed over the active region 20, and a p-type top mirror stack 24 (another DBR) is disposed over the top spacer 22. Over the top mirror stack 24 is a p-type conduction layer 9, a p-type GaAs cap layer 8, and a p-type electrical contact 26.
Still referring to FIG. 1, the lower spacer 18 and the top spacer 22 separate the lower mirror stack 16 from the top mirror stack 24 such that an optical cavity is formed. As the optical cavity is resonate at specific wavelengths, the mirror separation is controlled so as to resonant at a predetermined wavelength (or at a multiple thereof). At least part of the top mirror stack 24 includes an insulating region 40, formed by implanting ions (protons), that provides current confinement. Alternatively, the insulating region 40 can be formed using an oxide layer, for example, in accordance with the teachings of U.S. Pat. No. 5,903,588, which is incorporated by reference. In either case, the insulating region 40 defines a conductive annular central opening 42 that forms an electrically conductive path through the insulating region 40.
In operation, an external bias causes an electrical current 21 to flow from the p-type electrical contact 26 toward the n-type electrical contact 14. The insulating region 40 and the conductive central opening 42 confine the current 21 such that it flows through the conductive central opening 42 to the active region 20. Some of the electrons in the current 21 are converted into photons in the active region 20. Those photons bounce back and forth (resonate) between the lower mirror stack 16 and the top mirror stack 24. While the lower mirror stack 16 and the top mirror stack 24 are very good reflectors, some of the photons leak out as light 23 that travels along an optical path. Still referring to FIG. 1, the light 23 passes through the p-type conduction layer 9, through the p-type GaAs cap layer 8, through an aperture 30 in the p-type electrical contact 26, and out of the surface of the vertical cavity surface emitting laser 10.
It should be understood that FIG. 1 illustrates a common VCSEL structure, and that numerous variations are possible. For example, the doping can be changed (say, by providing a p-type substrate 12), different material systems can be used, operational details can be tuned for maximum performance, and additional structures, such as tunnel junctions, can be added.
While individual VCSELs are of great interest, some applications can benefit from arrays of VCSEL elements. For example, FIG. 2 illustrates a VCSEL array 60 comprised of four, evenly spaced, individual VCSEL elements 68, each of which could be in accord with FIG. 1. Many applications can also benefit from photodetector arrays that optically mate with VCSEL arrays. Turning now to FIG. 3, such a photodetector array 66 can be comprised of individual, evenly spaced, photodetectors 65. In practice, the individual VCSELs 68 of a VCSEL array 60, and the individual photodetectors of a detector array 66 are usually spaced the same distance apart, for example, 250 microns. While FIGS. 2 and 3 show 4 element arrays, in practice opto-electronic arrays can have different numbers of individual elements, with 12 element arrays being fairly common.
While generally successful, VCSEL arrays and matching photodetector arrays have their problems. One particular problem is interconnecting VCSEL arrays and/or photodetector arrays with higher-level systems. Such interconnections often require both electrical connections to another structure (such as a printed circuit board) and optical coupling with optical fibers. Common design requirements of electrical connections for high-speed communication applications include short lead length and bound wire length for well-controlled line input, termination impedances, and low parasitic capacitances.
Optically coupling VCSEL and/or photodetector array substrates to optical fibers present additional problems. For example, a precise physical alignment between VCSEL and/or photodetector array elements and optical fibers is often required. Indeed, in some applications the optical alignment must be within a micron or so. Prior art optical alignment techniques approaches include molded lens coupling, butt coupling, and butt coupling with V-groove alignment. In the cases of using fiber butt coupling techniques, fiber facets must be positioned to be very close to the active region of VCSEL or photodetector array chip, which leads to several undesired packaging limitations. It would be difficult or even almost impossible to form a hermetic package if a glass window is inserted. Furthermore, electrical bond pads are often forced to be positioned away from the active regions in order to make room for optical fiber interface. This leads to added cost as chip dimension increases and chip yield per wafer decreases. Moreover, parasitic capacitance increases as electrical lead length increases. Molded external plastic lens is also not a good typical solution for a hermetic package. Molded plastic lens array typically has a significantly higher coefficient of thermal expansion (hereinafter “CTE”). A large CTE mismatch between the VCSEL/photodetecter array, lens array and the fiber array can cause optical coupling efficiency variation among array elements, thereby limiting the operation temperature range of the assembly.
In view of the foregoing problems, a new technique of packaging opto-electronic semiconductor arrays would be beneficial. Even more beneficial would be a new packaging system having a submount for receiving opto-electronic semiconductor arrays, such as VCSEL or photo-detector arrays, such that those arrays interface with optical fibers. Even more beneficial would be an electrically connectable packaging assembly that facilitates electrical connections between opto-electronic semiconductor arrays and external circuitry, while providing for optical interfacing with optical fibers. Still more beneficial would be an electrically connectable packaging assembly that facilitates electrical connections between packaged opto-electronic semiconductor arrays and external circuitry, while providing for optical interfacing with optical fibers by way of a micro lens array. Beneficially, such a micro lens array would be easily producible in large quantities at a low cost and of high optical quality. Also beneficial would be a technique of forming such micro lens arrays by ejecting, such as by ink jet ejection, optical epoxy onto a transparent substrate, such as a glass substrate, for example. Still more beneficial would be such a lens array formed on a transparent substrate that can provide hermetical seal to opto-electronic devices and device arrays. Still more beneficial would be such a lens array formed on a transparent substrate that provides superior CTE match with opto-electronic device arrays and fiber array connectors such that there is more robust optical coupling over a wide operation temperature range between the array components. Still more beneficial would be a new technique of interconnecting arrays of semiconductor-based optical elements, such as VCSEL and/or photo detector arrays, with parallel optical fibers.